The present invention relates to an anode having an anode active material layer on an anode current collector, and a secondary battery including the anode.
In recent years, portable electronic devices such as video cameras, mobile phones, and notebook personal computers have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as an electric power source for the portable electronic devices, a battery, in particular a light-weight secondary battery capable of providing a high energy density has been developed.
Specially, a secondary battery using insertion and extraction of lithium for charge and discharge reaction (so-called lithium ion secondary battery) is extremely prospective, since such a secondary battery is able to provide a higher energy density than a lead battery and a nickel cadmium battery.
The lithium ion secondary battery includes a cathode, an anode, and an electrolytic solution. The anode has an anode active material layer on an anode current collector. The anode active material layer contains an anode active material contributing to charge and discharge reaction.
As the anode active material, a carbon material has been widely used. However, in recent years, as the high performance and the multi functions of the portable electronic devices are developed, further improvement of the battery capacity is demanded. Thus, it has been considered to use silicon instead of the carbon material. Since the theoretical capacity of silicon (4199 mAh/g) is significantly higher than the theoretical capacity of graphite (372 mAh/g), it is prospected that the battery capacity is thereby highly improved.
In the case where silicon is used as an anode active material, evaporation method is used as a method of forming an anode active material layer. In the evaporation method, the anode active material layer is linked to and united with an anode current collector, and thus the anode active material layer is less likely to expand and shrink in charge and discharge. However, in the case where silicon is deposited by using the evaporation method, there is concern that a silicon film becomes noncrystalline (amorphous). In the amorphous silicon film, the physical property is easily changed with time, and contact strength of the anode active material layer to the anode current collector is easily lowered by being affected by oxidation. Accordingly, the cycle characteristics, the charge and discharge characteristics and the like as important characteristics of the secondary battery may be lowered.
For using silicon as an anode active material, various technologies have been already proposed. Specifically, regarding a composition of an anode active material, a technique that an anode active material having silicon and a transition metal element as an element is used is known as described in, for example, Japanese Unexamined Patent Application Publication No. 2003-007295. Further, regarding a method of depositing an anode active material, a technique that particles primarily composed of silicon are not melted or evaporated but dispersed in air, and the surface of an anode current collector is sprayed with the dispersed particles, and thereby silicon is deposited is known as described in, for example, Japanese Unexamined Patent Application Publication No. 2005-310502. Furthermore, regarding a crystal state of an anode active material, for example, Japanese Unexamined Patent Application Publication No. 2002-083594 discloses a technique that amorphous or microcrystalline silicon is used and Japanese Unexamined Patent Application Publication No. 2007-194207 discloses a technique that crystalline (Raman shift is 490 cm−1 to 500 cm−1 and peak half-width is 10 cm−1 to 30 cm−1) silicon is used.